PIC16F88

Manufacturer Part NumberPIC16F88
ManufacturerMicrochip Technology Inc.
PIC16F88 datasheet
 
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Page 77/228:

THE TIMER1 OSCILLATOR

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7.6
Timer1 Oscillator
A crystal oscillator circuit is built between pins T1OSI
(input) and T1OSO (amplifier output). It is enabled by
setting control bit T1OSCEN (T1CON<3>). The oscilla-
tor is a low-power oscillator, rated up to 32.768 kHz. It
will continue to run during all power-managed modes.
It is primarily intended for a 32 kHz crystal. The circuit
for a typical LP oscillator is shown in Figure 7-3.
Table 7-1 shows the capacitor selection for the Timer1
oscillator.
The user must provide a software time delay to ensure
proper oscillator start-up.
Note:
The Timer1 oscillator shares the T1OSI
and T1OSO pins with the PGD and PGC
pins
used
for
programming
debugging.
When using the Timer1 oscillator, In-Circuit
Serial Programming™ (ICSP™) may not
function correctly (high voltage or low
voltage), or the In-Circuit Debugger (ICD)
may not communicate with the controller.
As a result of using either ICSP or ICD, the
Timer1 crystal may be damaged.
If ICSP or ICD operations are required, the
crystal should be disconnected from the
circuit (disconnect either lead) or installed
after programming. The oscillator loading
capacitors may remain in-circuit during
ICSP or ICD operation.
FIGURE 7-3:
EXTERNAL
COMPONENTS FOR THE
TIMER1 LP OSCILLATOR
C1
PIC16F87/88
33 pF
T1OSI
XTAL
32.768 kHz
T1OSO
C2
33 pF
Note:
See the Notes with Table 7-1 for additional
information about capacitor selection.
 2005 Microchip Technology Inc.
PIC16F87/88
TABLE 7-1:
CAPACITOR SELECTION FOR

THE TIMER1 OSCILLATOR

Osc Type
Freq
LP
32 kHz
Note 1: Microchip suggests this value as a starting
point in validating the oscillator circuit.
2: Higher capacitance increases the stability
of the oscillator but also increases the
start-up time.
3: Since each resonator/crystal has its own
characteristics, the user should consult
the resonator/crystal manufacturer for
appropriate
and
components.
4: Capacitor values are for design guidance
only.
7.7
Timer1 Oscillator Layout
Considerations
The Timer1 oscillator circuit draws very little power
during operation. Due to the low-power nature of the
oscillator, it may also be sensitive to rapidly changing
signals in close proximity.
The oscillator circuit, shown in Figure 7-3, should be
located as close as possible to the microcontroller.
There should be no circuits passing within the oscillator
circuit boundaries other than V
If a high-speed circuit must be located near the oscilla-
tor, a grounded guard ring around the oscillator circuit,
as shown in Figure 7-4, may be helpful when used on
a single-sided PCB or in addition to a ground plane.
FIGURE 7-4:
OSCILLATOR CIRCUIT
WITH GROUNDED
GUARD RING
C1
C2
33 pF
33 pF
values
of
external
or V
.
SS
DD
V
SS
OSC1
OSC2
RB7
RB6
RB5
DS30487C-page 75